Operational amplifiers have many applications in control and communication circuits. One particular application is in implementation of high performance integrators for sampled data systems.
Performance characteristics of operational amplifiers are chosen to optimize the operational amplifier's performance to its application. These characteristics include open loop voltage gain, settling time, common mode rejection, input and output impedance and input and output voltage swing. Modern systems typically operate at relatively low power supply voltages, such as 3 V rail to rail or less, and operational amplifiers need to operate at such voltages. Moreover, power dissipation in many circuits should always be minimized. For economical implementation, operational amplifiers should be relatively simple in design, requiring only small amounts of die area when included in an integrated circuit.
In sampled data systems, a particularly important performance characteristic is the settling time of the operational amplifier. The settling time is the time in which the output signal reaches a specified percentage (such as 0.024% for 12-bit resolution or 0.012% for 13 bit resolution) of its final value. The settling time in part establishes the maximum speed at which the operational amplifier can be operated. This in turn relates to the clock speed of a system with which the operational amplifier can be used.
Modern systems are being operated at higher clock speeds, creating needs for ever faster operational amplifiers. For example, in radio communication technology, the Global System for Mobile (GSM) communication standard is being extended from the 900 MHz frequency band to the 1.9 GHz band. At the same time, power dissipation requirements are growing more stringent and operating voltage for radios in such systems are being reduced to the 2-3 V range. Prior art operational amplifiers have not been able to meet these performance characteristics.
Accordingly, there is a need in the art for a low-power operational amplifier having fast settling time and high voltage gain suitable for use in sampled data systems.